Enhancement of the mechanical properties by graphite flake addition

ABSTRACT

Compositions in accordance with the invention comprise a polymer and flake reinforcing material distributed throughout the polymer in an effective amount to structurally reinforce the polymer. Individual flakes of the flake material (a) are less than or equal to 1,000 Angstroms in thickness, (b) have an aspect ratio greater than or equal to 100, and (c) are preferably significantly randomly oriented throughout the polymer. A novel apparatus for shear grinding a platy solid material into such individual flakes comprises a cylindrical shearing drum and a shear grinder received therein. The shearing drum has a longitudinal axis and an internal surface formed about a first predetermined radius of curvature. The cylindrical drum is supported for rotation about its longitudinal axis. The shear grinder has an external surface formed about a second predetermined radius of curvature. The second radius of curvature is slightly less than the first radius of curvature.

TECHNICAL FIELD

This invention relates generally to reinforcement of polymers, and toapparatus for shear grinding platy solid materials into thin flakes.

BACKGROUND ART

At present, there are hundreds of polymers that are used in a widevariety of products. These polymer materials differ in their molecularcomposition and construction, and consequently in their physicalproperties such as melting point, strength, stiffness, etc. Themolecular level differences produce differences in the way in whichthese materials must be processed in order to produce useful finishedproducts. Generally higher performance polymers capable of exposure toextreme environments are expensive and relatively difficult to shape.

Polymers can be used in their relatively pure form in producing usefularticles, or also as the matrix phase in composite materials. Suchcomposite materials typically comprise a matrix phase and areinforcement phase. With most composites, it is desired that thefinished material possess structural and other mechanical propertiestypical of metal.

To achieve the desired structural properties, the reinforcement phase istypically maximized and is comprised of a fibrous reinforcing material.The orientation of the reinforcement fibers is usually controlled toproduce the highest strength and stiffness of the finished material inthe desired direction or directions. Various filler materials can alsobe added to pure polymers or polymer composites to increase durability,or simply to decrease the amount of the relatively more expensivepolymer which is used. These fillers are usually roughly spherical inshape, and do not function well as reinforcements because the sphericalshape does not allow much load transfer by shear.

Graphite fiber is one material that has been added to polymers forreinforcement. Examples of finished products incorporating polymers andgraphite fibers are golf club shafts, fishing rods, etc. The graphitematerial in such products is typically comprised of long fibers whichare woven and specifically oriented within the material to maximizestrength or flexibility in a given direction.

The primary intent of this invention was to create stronger polymercomposite materials at reduced cost. Other advantages may be achievableby practice of the various aspects of the invention as will beappreciated by the artisan.

BRIEF DESCRIPTIONS OF THE DRAWINGS

A preferred embodiment of one aspect of the invention is illustrated inthe accompanying drawings, in which:

FIG. 1 is a perspective view of an apparatus for shear grinding of platysolid material into thin flakes in accordance with one aspect of theinvention. The apparatus is illustrated in an operating rotationalstate.

FIG. 2 is a cross-sectional view taken along line 2--2 in FIG. 1 as theapparatus is rotating.

FIG. 3 is a cross-sectional view as would typically appear if takenalong line 2--2 when the apparatus was in a stationary, non-rotationalstate.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS AND BEST MODES FORCARRYING OUT THE INVENTION

The following disclosure of the invention is submitted in compliancewith the constitutional purpose of the Patent Laws "to promote theprogress of science and useful arts" (Article 1, Section 8).

Compositions in accordance with the invention comprise a polymer and aflake reinforcing material distributed therethrough in an effectiveamount to structurally reinforce the polymer. Individual flakes of theflake material (a) are less than or equal to 1,000 Angstroms inthickness, (b) have an aspect ratio (largest dimension/smallestdimension) greater than or equal to 100, and (c) preferably aresignificantly randomly oriented throughout the polymer as opposed tobeing deliberately aligned or configured in some organized manner. It isanticipated that any of a wide variety of platy solid materials that canbe ground into thin flakes can function as the flake reinforcingmaterial. Examples of such materials include graphite, mica and talcflakes. Further examples of workable materials might include glassflakes, certain platy clays such as kaolinite, and certain platy metalores such as MoS₂ and ZnS. Actual reduction to practice at this writinghas been demonstrated with graphite flakes.

The effective amount of flake reinforcing material in the composition ispreferably anywhere from 5 to 50 volume percent, with volume percents inthe lower portion of this range being most preferred. The thickness ofindividual flakes will typically fall between 100 and 1,000 Angstroms,with a flake thickness of less than or equal to 500 Angstroms beingpreferred. Graphite flake is the preferred reinforcing flake materialadditive in terms of its strength, stiffness, low density and low cost.It is anticipated that with a 10 volume percent inclusion of graphiteflakes in inexpensive polymers, such as polyethylene or polypropylene,the stiffness of the finished product will approach that of aluminum.Although graphite flakes are very strong along two directions, they arevery weak and subject to shear in a third. Accordingly, the flakesshould be as thin as possible to preclude development of large shearforces across them in use.

Similarly, the flakes should typically not be deliberately aligned inthe polymer, but rather should be significantly randomized to avoidweakness in the finished product in any one direction. As "randomness"is a matter of degree, the terminology "significantly random" is used.This terminology is intended to imply any degree of flake randomnesshaving a desired effect of contributing to strength and isotropy in thefinished material. If anisotropy in the finished material were desired,the flakes could be suitably oriented to produce the desired propertiesin the desired directions in the finished composite.

Thin graphite flakes less than 1,000 Angstroms in thickness and havingan aspect ratio in excess of 100 could be produced by a number ofdifferent methods. For example, the anisotropy in the strength of agraphite crystal makes it relatively easy to grind into thin flakes byball-milling techniques. For greatest efficiency, a simple andrelatively volatile hydrocarbon, such as octane, is preferably usedduring milling. The purpose of the hydrocarbon is to cover newly exposedsurfaces that result from the shearing to prevent possible recombinationof individual flakes. Natural graphite has been ball milled into flakes400 Angstroms thick and 7 microns in the remaining two dimensions, whichprovides an aspect ratio of approximately 175. Other methods, such asultrasonic vibration or by an exfoliation/intercalation process mightalso be usable in producing thin high aspect ratio flakes for use inaccordance with the invention. An inventive method and apparatus forproducing such thin flakes will be described below.

Structural reinforcement of polymers by graphite flake addition has beendemonstrated. Two identical size cantilever beams were fabricated fromDER 332 epoxy resin, manufactured by Dow Chemical. One beam consistedessentially of the pure epoxy, while the other contained approximately10 to 11 volume percent of randomly distributed graphite flakes that hadbeen ball-milled to approximately 450 Angstroms thick, and having anaspect ratio of approximately 170. When the two cantilever beams weremechanically loaded with weights, there was a dramatic difference in thedeflections. Using the measured deflections, it was calculated that thegraphite flake reinforced beam was 42 times as stiff as thenon-reinforced beam.

FIGS. 1-3 diagrammatically illustrate a shear grinding apparatus 10useful for grinding a platy material 11 into thin, high aspect ratioflakes. Apparatus 10 comprises a cylindrical shearing drum 12 having alongitudinal axis 14. Shearing drum 12 includes a cylindrical internalsurface 16 formed about a first predetermined radius of curvaturerepresented by arrow 18 (FIG. 2). Shearing drum 12 would be mounted orotherwise supported (not shown) for rotation about its longitudinal axis14.

A shear grinder 20 is freely received within cylindrical drum 12, andrests on a bottom internal surface thereof. Shear grinder 20 has anexternal surface formed about a second predetermined radius Ofcurvature, with the second radius of curvature being slightly less thanthe first radius of curvature. Both shear grinder 20 and the internalsurface of drum 12 would be constructed of a hard material, such asalumina. The external surface of grinder 20 and internal surface of drum12 would be ground smooth during manufacture, and polished even smootherduring grinding operation.

More particularly, shear grinder 20 is solid and generallyhemicylindrical, having external surfaces defined by an arcuate surface22 and a non-arcuate, or planar, surface 24. Arcuate surface 22 has asecond predetermined radius of curvature, indicated by arrow 26 (FIG.2), and joins with planar surface 24 at first and second generallydiametrically opposed locations 28, 30. Cylindrical drum 12 is mountedfor rotation in a first direction `A`. First location 28 where arcuatesurface 22 joins with planar surface 24 precedes second location 30 indirection of rotation `A`. Arcuate surface 22 includes a tapered orrecessed leading nose portion 32 at location 28. Its purpose isdescribed below. Grinder 20 is preferably freely received within drum 12or isolated from rotating therewith to enable sliding of surfaces 16 and22 against one another.

FIG. 3 illustrates the relationship of drum 12 and shear grinder 20 in aresting position where shearing drum 12 is not rotating. FIGS 1 and 2illustrate the relationship of drum 12 and shear grinder 20 when drum 12rotates in direction `A` at some predetermined rotational speed. As drum12 rotates in direction `A`, shear grinder 20 tends to ride-up alonginternal drum surface 16 until some angle theta from horizontal isreached. Angle theta is a function of the friction coefficient and speedof the system for a given material that is being sheared. At angletheta, the mass of the shear grinder overcomes the frictionalrelationship of the system, which causes shear grinder 20 to effectivelyslide relative to the moving internal surface 16 of drum 12. This shearsthe graphite or other platy material 11 trapped between external surface22 of shear grinder 20 and internal surface 16 of drum 12. The greatestor most efficient shearing for a given batch is expected to occur at ornear the maximum rotational speed of drum 12 where shear grinder 20still does not rotate with drum 12.

As illustrated, shear grinder 20 is preferably hemicylindrical asopposed to being completely cylindrical. This lowers the center ofgravity of the shear grinder and minimizes any tendency of the grinderto roll with shearing drum 12, as opposed to sliding with respectthereto. A tapering recessed portion is preferably provided in thegrinder when the grinder is less than entirely cylindrical. The recessedportion, such as nose 28 in grinder 20, is positioned in the directionof anticipated rotation at the location where the arcuate cylindricalsurface deviates from its standard curvature. The recess facilitatesdrawing of the platy material into the sliding grinding area between thedrum and grinder as the drum rotates.

As mentioned above, the radius of curvature of external surface 22 ofgrinder 20 is preferably only slightly less than the radius of curvatureof internal surface 16 of drum 12. This will provide a rather largeshear grinding area of contact between drum 12 and grinder 20. Mostpreferably, the grinder radius is at least 99.0 percent of the insideradius of the drum. For example, a grinder radius of about 6.0 incheswould preferably be employed with a drum having a radius of 6.005inches. Where the difference between the two radiuses of curvature issignificantly greater than this, the area of contact between the drumand grinder reduces to a mere line of contact as opposed to an area.This adversely affects the grinding efficiency and will tend to enablethe grinder to flop around inside the drum as it rotates. Constructionswith a grinder radius less than 99.0 percent of the drum radius mightalso be possible without departing from the principles and scope of theinvention. However, an adverse effect on efficiency should beanticipated.

Apparatus in accordance with this aspect of the invention enableshearing of platy material along its planes of weakness into thin flakesof high aspect ratio. As with the ball-milling technique describedabove, a simple and relatively volatile hydrocarbon is preferablycombined with the platy material (at least preferably with graphite)when shear grinding.

In compliance with the statute, the invention has been described inlanguage more or less specific as to structural and compositionalfeatures. It is to be understood, however, that the invention is notlimited to the specific aspects shown and described, since the means andconstruction herein disclosed comprise preferred forms of putting theinvention into effect. The invention is, therefore, claimed in any ofits forms or modifications within the proper scope of the appendedclaims, appropriately interpreted in accordance with the doctrine ofequivalents.

I claim:
 1. A composition of improved strength comprising:a polymer; anda flake reinforcing material distributed throughout the polymer in aeffective amount to significantly structurally reinforce the polymer,individual flakes of the flake reinforcing material(a) being less thanor equal to 1000 Angstrom in thickness, (b) having an aspect ratiogreater than or equal to 100, and (c) being significantly randomlyoriented throughout the polymer; the composition being of significantlyhigher strength than the polymer without such flake reinforcingmaterial.
 2. The composition of claim 1 wherein the flake reinforcingmaterial comprises graphite.
 3. The composition of claim 1 wherein theeffective amount of flake reinforcing material is from 5 to 50 volumepercent.
 4. The composition of claim 1 wherein the thickness ofindividual flakes of the flake reinforcing material is betweenapproximately 100 and 1000 Angstroms.
 5. The composition of claim 1wherein the thickness of individual flakes of the flake reinforcingmaterial is less than or equal to 500 Angstroms.
 6. The composition ofclaim 1 wherein the flake reinforcing material comprises graphite whichis present in an effective amount of from 5 to 50 volume percent.
 7. Thecomposition of claim 6 wherein the thickness of individual flakes of theflake reinforcing material is between approximately 100 and 1000Angstroms.
 8. The composition of claim 6 wherein the thickness ofindividual flakes of the flake reinforcing material is less than orequal to 500 Angstroms.
 9. The composition of claim 1 wherein theeffective amount of flake reinforcing material is from 5 to 50 volumepercent, and the thickness of individual flakes of such reinforcingmaterial is between approximately 100 and 1000 Angstroms.
 10. Thecomposition of claim 1 wherein the flake reinforcing material comprisesmica.
 11. The composition of claim 10 wherein the effective amount ofmica is from 5 to 50 volume percent.
 12. The composition of claim 10wherein the thickness of individual mica flakes is between approximately100 and 1000 Angstroms.
 13. The composition of claim 10 wherein thethickness of individual mica flakes is less than or equal to 500Angstroms.
 14. The composition of claim 10 wherein,the effective amountof mica is from 5 to 50 volume percent; and the thickness of individualmica flakes is less than or equal to 500 Angstroms.
 15. The compositionof claim 1 wherein the flake reinforcing material is selected from thegroup consisting of glass flakes, clay flakes produced from platy clays,flakes produced from metal ores, and talc flakes.
 16. A method forimproving the strength of a polymer comprising:combining a polymer withan effective amount of a flake reinforcing material to form a mixture,the flake reinforcing material being significantly randomly orientedthroughout the mixture, the effective amount of flake reinforcingmaterial being sufficient to significantly structurally reinforce thepolymer, individual flakes of the flake reinforcing material(a) beingless than or equal to 1000 Angstroms in thickness, and (b) having anaspect ratio greater than or equal to 100; and allowing the mixture toharden into a solid reinforced polymer product.
 17. The method of claim16 wherein the effective amount of flake reinforcing material is from 5to 50 volume percent.
 18. A reinforced polymer produced according to themethod of claim
 16. 19. A reinforced polymer produced according to themethod of claim 17.